build: fix lack of AC_LANG_SOURCE warnings at autogen time

configure.ac:112: the top level
configure.ac:693: warning: AC_LANG_CONFTEST: no AC_LANG_SOURCE call detected in body
../../lib/autoconf/lang.m4:193: AC_LANG_CONFTEST is expanded from...
../../lib/autoconf/general.m4:2590: _AC_COMPILE_IFELSE is expanded from...
../../lib/autoconf/general.m4:2606: AC_COMPILE_IFELSE is expanded from...

solution is to check compiler features via CPP (faster than compiling code)
and define AC_LANG_SOURCE for libnet build test.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>